Alterations in the DNA of the genome, specified by the efficiency and accuracy of DNA repair, have been considered as a major cause of aging. Our recent work has been focused on the development of transgenic mouse models to quantitate and characterize mutations in various organs and tissues as a function of age. This has resulted in a mouse model harboring the lacZ reporter gene as part of a chromosomally integrated plasmid vector that can be easily recovered in large numbers and inspected for mutations in E.coli, using a positive selection system. With this system, we demonstrate a spontaneous mutation frequencies of about 5 x 10/-5 per locus, above the E. coli background, in young animals. Most type of mutations were detectable, from point mutations to large deletions up to 13 cM (about 26 megabase). During aging these frequencies were found to increase in some organs and not in others. A natural next step in this study and the long term objective of this project is to study age-related organ and tissue specific patterns of somatic mutations in DNA repair deficient mouse models in relation to various markers of cellular aging. The hypotheses to be studied are that organ-specific mutation frequencies and spectra are specified by DNA repair status and are a causal factor in cellular senescence. The specific aims of the proposal are (1) to determine mutation frequencies an spectra in mice with deficiencies in nucleotide excision repair, base excision repair, recombinatorial repair or post-replication repair (2) to study mutation frequencies in mouse and human primary cell cultures as a function of in vitro life span; and (3) to generate new transgenic mice with an expressed reporter gene.